Ozone is utilized in a number of industrial processes, including drinking water and waste water treatment and disinfection, pulp bleaching, ozonolysis reactions in fine chemical production, and flue-gas denitrification.
Ozone is an unstable compound that decomposes to oxygen under ambient conditions and therefore it is not feasible to manufacture, transport or store in the manner used for many chemicals supplied through normal commerce. Rather, ozone must be produced at the point-of-use and at the time it is needed. Since ozone is a toxic material, it is generated only where and when it is required, in order to limit the possibility and potential impact of incidents.
Ozone is typically generated from oxygen utilizing a corona discharge. Oxygen is often used as the oxygen source for ozone generation and results in ozone concentrations of 10 to 15% by weight (balance oxygen) being produced. Air may also be used as the source of oxygen and produces ozone concentrations of 1.5 to 3% (balance air). For moderate to large ozone requirements, the total capital plus operating costs are typically less when oxygen is used as the oxygen source.
Ozone is often utilized at 10 weight % ozone with the balance being largely oxygen. It has been recognized that the re-use of the oxygen from the ozone/oxygen mixture generated by oxygen-based ozone generators can substantially improve the economics for ozone generation.
Adsorption systems such as those marketed as OZORA™ by Linde AG can be subject to severe perturbations resulting from changes in mode and where desorbing gas can originate from two or more sources. The OZORA™ system is a four bed adsorption system designed to recover oxygen from mixtures of ozone and oxygen streams. The adsorbent is selected to preferentially adsorb ozone, while allowing oxygen to pass through the adsorbent. The oxygen is either re-used by recycling it back to the inlet of the ozone generator or is used to rinse impurities from an adsorbent bed. The OZORA™ oxygen recovery system is unique in that three gas streams must be managed: ozone/oxygen, recycled oxygen and clean dry air (CDA)/ozone product for customer use.
The OZORA™ system further employs a unique bed-to-bed gas transfer, rinse step to rinse the CDA from a recently desorbed bed and transfer it to a receiving adsorbent bed for the initial desorption step. Prior to the present invention, the bed-to-bed transfer of CDA was unregulated and uncontrollable. The percentage of flow attributable to the rinse step was solely dependent on the difference in pressure drops across the two gas paths.
For the OZORA™ system, the ability to regulate at least one of the above-mentioned flow paths is critical in controlling the proportioning of gas flow paths. The rinse step is critical in preparing a bed for the adsorption step by flushing nitrogen from that bed. The rinse step flushes residual nitrogen from a bed to control the purity of the recycled oxygen. The time required for this step is dependent on the flow rate, which is critical for a system that uses timing cycles. The time of the rinse step is dependent on the flow rate for that portion of flow diverted from the oxygen recycle stream to the rinse stream.